Extract workedexample directive to new Python package if generalized

[jpmckinney]

If there is a need for a similar way to tag and list content, then we should:

• Rename the directives to something like taggedcontent and taggedcontentlist and add an option category in addition to tag. That way, we can set the category to workedexample or usecase or whatever else.
• Change the taggedcontentlist (workedexamplelist) directive to just render the bullet list. We can then use that directive anywhere we like, whether in an admonition:

````{admonition} Worked examples
:class: note

Please refer to the worked examples relating to awards and contracts:

```{taggedcontentlist} workedexample awardscontracts
```
````

Or in the body of the page:

Please refer to the worked examples relating to awards and contracts:

```{taggedcontentlist} workedexample awardscontracts
```